Fiber preforming process and coating for screen tooling therefor

ABSTRACT

An improved screen tooling for a fiber preforming process, and a durable coating system for such tooling. The coating system comprises a surface layer of a porcelain enamel composition that is generally a borosilicate glass, and preferable contains quartz, borax, boric oxide, potassium nitrate, sodium silicofluoride, and manganese dioxide, and optionally contains titanium dioxide, antimony oxide, cobalt oxide and/or barium oxide. Preferred compositions are dependent in part on the screen tooling material. The invention also encompasses a fiber preforming process that utilizes screen tooling with the coating system.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division patent of U.S. patent application Ser. No.09/638,010, filed Aug. 14, 2000 now U.S. Pat. No. 6,610,229.

BACKGROUND OF INVENTION

The present invention generally relates to fiber preforming methods andtooling used to form fiber preforms. More particularly, this inventionrelates to screen tooling for a fiber preforming process and a coatingsystem therefor, in which the coating system inhibits sticking of fiberpreforms to the tooling and promotes the tooling life of the screentooling.

Various methods are known for forming fiber preforms that are suitablefor producing fiber-reinforced articles, such as glass-reinforcedpolymer composite beds for pickup trucks. One such method is referred toas directed fiber preforming, which makes use of a perforated screentooling having a surface whose shape corresponds to that of the desiredfiber-reinforced article. In this process, reinforcement fibers (typicalglass fibers) are sprayed onto the surface of the screen tooling andheld on the tooling surface by a vacuum drawn through the tooling. Thefibers are then bonded together with a binder to yield a porous fiberpreform having a fixed shape corresponding to that of the toolingsurface. The binder may be pre-coated on the fibers, simultaneouslysprayed with the fibers onto the tooling, or sprayed on the fibers whilethe fibers are held on the tooling under vacuum. After curing, the fiberpreform is removed from the screen tooling and placed in a suitable moldinto which a resin is injected to infiltrate the preform. The resin isthen cured to yield the fiber-reinforced article.

The release of the preform from the screen tooling is a processing issuefor directed fiber preforming processes. For the preform to maintain itsintegrity, it must release cleanly from the tooling after the binder hasbeen cured. However, in the process of bonding the fibers together, thebinder also tends to adhere the preform to the tooling. In the past,screen tooling has been coated with a release agent or a semi-permanentcoating such as polytetrafluoroethylene (PTFE, or TEFLON®) to inhibitthe preform from adhering to the tooling. While suitable for manyapplications, release agents must typically be reapplied after eachpreform operation, and may adversely effect the properties of thepreform. TEFLON® coatings are not sufficiently durable to survivenumerous molding operations, and therefore require significantproduction downtime to repair the coating or completely recoat thescreen tooling.

In view of the above, it would be desirable if an improved screentooling and coating system were available that was more durable, reducedor eliminated the requirement for release agents, and extended theservice life of the tooling.

SUMMARY OF INVENTION

The present invention is directed to an improved screen tooling for afiber preforming process, and more particularly to a durable coatingsystem for such tooling. The coating system of this invention comprisesa surface layer of a porcelain enamel composition that is generally aborosilicate glass, and preferable contains quartz (SiO₂), dehydratedborax (Na₂B₄O₇), boric acid (H₃BO₃), potassium nitrate (KNO₃), sodiumsilicofluoride (Na₂SiF₆), and manganese dioxide (MnO₂), and optionallycontains titanium dioxide (TiO₂), antimony oxide (Sb₂O₃), cobalt oxide(cobaltous oxide (CoO), cobalto-cobaltic oxide (Co₃O₄) and/or cobalticoxide (Co₂O₃)) and/or barium oxide (BaO). Preferred compositions aredependent in part on the screen tooling material. The invention alsoencompasses a fiber preforming process that utilizes screen tooling withthe coating system.

According to the present invention, continuous coatings formed ofporcelain enamel compositions have been shown to be more durable thansemi-permanent coatings currently in use, and therefore require lessmaintenance such that production cost and downtime are reduced. Inaddition, it has been determined that preforms release more readily andcleanly from screening tooling protected by the coating system of thisinvention, such that the quality of the fiber preform and therefore thefinal fiber-reinforced article is promoted. The use of release agentscan be reduced or eliminated with the coating system, thereby decreasingthe preform process cycle time and reducing the likelihood that therelease agent will degrade the preform properties.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically represents a perforated screen tooling with acontinuous outer porcelain enamel coating in accordance with the presentinvention, on which a fiber preform is formed by a directed fiberpreforming processes.

FIG. 2 schematically represents an apparatus for molding an articlereinforced with the directed fiber preform produced with the perforatedscreen tooling of FIG. 1.

DETAILED DESCRIPTION

Tooling 10 for a fiber preforming process is represented in FIG. 1,along with a fiber preform 12 produced with the tooling 10. The tooling10 is represented as a perforated screen of the type used in directedfiber preforming processes, in which reinforcement fibers, e.g., glassfibers, are sprayed or otherwise deposited on the covex tooling surface14 of the screen tooling 10, and thereafter held on the tooling surface14 by a vacuum drawn through the tooling 10 from the opposite side 16 ofthe tooling 10. The thickness and uniformity of the layer of fibers onthe tooling 10 will depend in part on the shape of the tooling surface14, the vacuum, and the physical properties of the fibers. A binder iscoated on the fibers prior to deposition, or simultaneously sprayed withthe fibers onto the tooling surface 14, or sprayed on the fibers whilethe fibers are held on the tooling 10 under vacuum. According to oneknown technique, the screen tooling 10 with the mass of fibers held atits surface 14 is then rotated into an oven where the binder is cured,yielding the porous fiber preform 12 whose shape corresponds to thesurface 14 of the tooling 10. Thereafter, the fiber preform 12 isremoved from the screen tooling 10 and placed in a suitable mold 18,which is represented in FIG. 2. A resin is injected into the mold 18 toinfiltrate the preform 12. The resin-impregnated fiber preform 12 isthen heated to cure the resin and thereby yield the desiredfiber-reinforced article. Those skilled in the art will appreciate thatFIGS. 1 and 2 are merely for illustrative purposes, in that the shape ofthe tooling 10, and therefore the shape of the preform 12, can differsignificantly from that shown.

According to the present invention, the screen tooling 10 is providedwith a coating system on at least its outer surface to prevent or atleast inhibit the fibers and binder from adhering to the tooling 10after the curing process. The coating system of this invention includesa porcelain (vitreous) enamel coating that is preferably continuous onthe surface 14 of the tooling 10. One or more intermediate layers couldbe used to promote the adhesion between the enamel coating and thesurface of the tooling 10. The composition of the coating is formulatedto be abrasion, heat and corrosion-resistant and inhibit the preform 12from adhering to the tooling 10. The material of the tooling 10preferably has a low free carbon content in order to reduce thelikelihood of blistering and poor adhesion of the coating to thetooling. For this purpose, low carbon steels are suitable for thetooling 10, though other materials having a low free carbon contentcould be used, such as steels with about 0.3 wt. % titanium and about0.15 wt. % silicon to maintain free carbon at very low levels.

According to the present invention, the porcelain enamel coating isgenerally a borosilicate glass composition that has been fused to thesurface of the screen tooling 10 by depositing a slip having anappropriate composition on the tooling 10, and then firing at anelevated temperature. Borosilicate coatings of the type required by thisinvention are always highly complex in their formulation, with physicaland mechanical properties that are determined principally by theircomposition. In turn, the coating composition must be carefully selectedand evaluated to ensure compatibility with the composition of thesubstrate, the manner in which the coating is deposited, the desiredfunction of the coating, and the service conditions that the coatingmust withstand.

The porcelain composition employed by the present invention is formed byfiring a slip composition containing quartz (SiO₂), dehydrated borax(Na₂B₄O₇), boric acid (H₃BO₃), potassium nitrate (KNO₃), sodiumsilicofluoride (Na₂SiF₆), and manganese dioxide (MnO₂), and optionallycontains titanium dioxide (TiO₂), antimony oxide (Sb₂O₃), cobalt oxide(cobaltous oxide (CoO), cobalto-cobaltic oxide (Co₃O₄) and/or cobalticoxide (Co₂O₃)) and/or barium oxide (BaO). Suitable ranges for theprefired constituents of the coating composition are, in weight percent,about 39 to 52 quartz, about 15 to 24 dehydrated borax, about 6 to 12boric acid, about 5 to 8 potassium nitrate, about 3 to 6 sodiumsilicofluoride, about 3 to 12 manganese dioxide, up to 15 titaniumdioxide, up to 3 antimony oxide, up to 1 cobalt oxide, and up to 1barium oxide. The dry constituents are mixed with water to form anaqueous dispersion of the dry constituents as a slip, in accordance withknown practices. The slip can then be deposited by air or electrostaticliquid spray on iron and steel substrates, and fired at about 750° C. to900° C. to yield a coating whose thickness is about 75 to 150micrometers. The final composition of the coating will depend in part onthe firing conditions, but will include the above-noted dry constituentsof the slip with the exception of boric oxide (B₂O₃) derived from theboric acid component of the slip. Suitable constituent ranges for thefinal coating are, in weight percent, about 39 to 52 quartz, about 15 to24 borax, about 7 to 12 boric oxide, about 5 to 12 potassium nitrate,about 3 to 8 sodium silicofluoride, about 3 to 12 manganese dioxide, upto 12 titanium dioxide, up to 8 antimony oxide, up to 1 cobalt oxide,and up to 1 barium oxide.

An investigation was undertaken directed to evaluating the releasecapability and the durability of coatings on steel screens. The screenswere perforated 2×4 ft. (about 0.6×1.2 m) P20 steel with about 40% openarea. A first group of screens was coated with a slip whose dryconstituents consisted of, in weight percent, about 46.5 quartz, about21 dehydrated borax, about 7.5 boric acid, about 6 potassium nitrate,about 5 sodium silicofluoride, about 11.5 manganese dioxide, and about2.5 antimony oxide. The screens were fired at a temperature of about880° C. to yield a continuous porcelain enamel coating having athickness of about 150 micrometers. Other screens of identicalconstruction and composition were not coated, or were painted with alayer of PTFE, or were plated with a layer of a nickel-phosphorous-PTFEalloy having a thickness of about 50 micrometers. The screens coatedonly with PTFE served as baselines for the investigation. A few of eachtype of specimen were evaluated with a release agent: an organic metalworking fluid available from Quaker Chemicals under the name DRAW 58, awax compound available from ChemTrend under the name CT2044, or a PTFEemulsion available from ChemTrend under the name CT88WA. The releaseagents were chosen based on their effectiveness to release a preformfrom an uncoated screen without causing molding problems or degradingthe bulk mechanical properties of the preform.

The preforms were formed of a chopped glass fiber material that wascoated with a polyester emulsion available from Codo Corporation andPolymers under the name 44-7013. While the binder was cured at about204° C. with a peroxide curing agent, the fiber material was held to thesurfaces of the screens under a vacuum of about 102 mbar. Followingcure, the preforms were removed from the screens, and the screens wereprepared as necessary for reuse by cleaning with acommercially-available chemical stripping agent to remove any residualbinder and reapplying the release agent (if used). The production of atleast 150 preforms per screen was targeted for the investigation.

The following observations were made during this investigation. Thebaseline uncoated screens did not show signs of wear, but requiredfrequent application of a mold release and frequent cleaning. Thescreens provided with only a PTFE coated performed well in terms of easeof removing the preforms, but the coatings showed signs of wear afteronly fifteen screens. Even with gentle cleaning, the PTFE coatings hadto be removed and replaced twice during the first fifty moldingoperations. As a result, the evaluation of the PTFE-coated screens wasterminated before completing 150 preforms. Those screens coated with thenickel-phosphorous-PTFE alloy also performed better than the baselinescreens, but showed signs of wear and required frequent application ofthe mold release agent to extract the preforms. Finally, from thestandpoint of the ability to easily remove preforms without damaging thepreforms and without repairing the coating, the screens provided withthe porcelain enamel coating of this invention performed better than anyof the others tested, regardless of whether the screens were coated witha release agent. From this investigation, it was concluded that theporcelain enamel coating combined the robustness of the uncoated screenswith the release capability of the PTFE-coated screens, while achievinglower processing costs.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Accordingly, the scope of the invention is to belimited only by the following claims.

1. A screen tooling for a fiber preforming process, the screen toolinghaving a coating system fused to a surface thereof, the coating systemcomprising a surface layer of a porcelain enamel composition, whereinthe porcelain enamel composition comprises quartz, borax, boric oxide,potassium nitrate, sodium silicofluoride and manganese dioxide, theporcelain enamel composition optionally comprising compounds chosen fromthe group consisting of titanium dioxide, antimony oxide, cobalt oxide,and barium oxide.
 2. The screen tooling according to claim 1, whereinthe porcelain enamel composition comprises, in weight percent, about 39to 52 quartz, about 15 to 24 borax, about 7 to 12 boric oxide, about 5to 12 potassium nitrate, about 3 to 8 sodium silicofluoride, about 3 to12 manganese dioxide, up to 12 titanium dioxide, up to 8 antimony oxide,up to 1 cobalt oxide, and up to 1 barium oxide.
 3. The screen toolingaccording to claim 1, wherein the porcelain enamel composition is firedfrom a dry mixture comprising quartz, dehydrated borax, boric acid,potassium nitrate, sodium silicofluoride, and manganese dioxide, the drymixture optionally comprising compounds chosen from the group consistingof titanium dioxide, antimony oxide, cobalt oxide, and barium oxide. 4.The screen tooling according to claim 1, wherein the porcelain enamelcomposition is fired from a dry mixture comprising, in weight percent,about 39 to 52 quartz, 15 to 24 dehydrated borax, 6 to 12 boric acid, 5to 8 potassium nitrate, 3 to 6 sodium silicofluoride, and 3 to 12manganese dioxide, up to 15 titanium dioxide, up to 3 antimony oxide, upto 1 cobalt oxide, and up to 1 barium oxide.
 5. The screen toolingaccording to claim 1, wherein the porcelain enamel composition is firedfrom a dry mixture comprising, in weight percent, about 46.5 quartz,about 21 dehydrated borax, about 7.5 boric acid, about 6 potassiumnitrate, about 5 sodium silicofluoride, and about 11.5 manganesedioxide.
 6. The screen tooling according to claim 5, wherein the drymixture further comprises about 2.5 antimony oxide.
 7. The screentooling according to claim 1, wherein the screen tooling is a perforatedmember, the coating system being continuous over the surface of theperforated member.
 8. The screen tooling according to claim 7, whereinthe coating system consists of the surface layer of the porcelain enamelcomposition.
 9. A screen tooling installed in a directed fiberpreforming apparatus, the screen tooling comprising a perforated memberand a continuous outer coating fused to a tooling surface thereof, thecoating consisting essentially of a porcelain enamel composition. 10.The screen tooling according to claim 9, wherein the porcelain enamelcomposition comprises quartz, borax, boric oxide, potassium nitrate,sodium silicofluoride and manganese dioxide, the porcelain enamelcomposition optionally comprising compounds chosen from the groupconsisting of titanium dioxide, antimony oxide, cobalt oxide, and bariumoxide.
 11. The screen tooling according to claim 9, wherein theporcelain enamel composition comprises, in weight percent, about 39 to52 quartz, about 15 to 24 borax, about 7 to 12 boric oxide, about 5 to12 potassium nitrate, about 3 to 8 sodium silicofluoride, about 3 to 12manganese dioxide, up to 12 titanium dioxide, up to 8 antimony oxide, upto 1 cobalt oxide, and up to 1 barium oxide.
 12. The screen toolingaccording to claim 9, wherein the porcelain enamel composition is firedfrom a dry mixture comprising quartz, dehydrated borax, boric acid,potassium nitrate, sodium silicofluoride, and manganese dioxide, andoptionally titanium dioxide, antimony oxide, cobalt oxide, and/or bariumoxide.
 13. The screen tooling according to claim 9, wherein theporcelain enamel composition is fired from a dry mixture comprising, inweight percent, about 39 to 52 quartz, 15 to 24 dehydrated borax, 6 to12 boric acid, 5 to 8 potassium nitrate, 3 to 6 sodium silicofluoride,and 3 to 12 manganese dioxide, up to 15 titanium dioxide, up to 3antimony oxide, up to 1 cobalt oxide, and up to 1 barium oxide.
 14. Thescreen tooling according to claim 9, wherein the dry mixture comprises,in weight percent, about 46.5 quartz, about 21 dehydrated borax, about7.5 boric acid, about 6 potassium nitrate, about 5 sodiumsilicofluoride, and about 11.5 manganese dioxide.
 15. The screen toolingaccording to claim 14, wherein the dry mixture further comprises about2.5 antimony oxide.
 16. A screen tooling for a fiber preforming process,the screen tooling having a coating system fused to a tooling surfacethereof, the coating system comprising a surface layer of a porcelainenamel composition comprising quartz, borax, boric oxide, potassiumnitrate, sodium silicofluoride and manganese dioxide, the porcelainenamel composition optionally comprising compounds chosen from the groupconsisting of titanium dioxide, antimony oxide, cobalt oxide, and bariumoxide.
 17. The screen tooling according to claim 16, wherein theporcelain enamel composition comprises, in weight percent, about 39 to52 quartz, about 15 to 24 borax, about 7 to 12 boric oxide, about 5 to12 potassium nitrate, about 3 to 8 sodium silicofluoride, about 3 to 12manganese dioxide, up to 12 titanium dioxide, up to 8 antimony oxide, upto 1 cobalt oxide, and up to 1 barium oxide.
 18. The screen toolingaccording to claim 16, wherein the surface layer is free of a releaseagent.
 19. The screen tooling according to claim 16, wherein the screentooling is a perforated member, the coating system being continuous overthe tooling surface of the perforated member.